In a variety of coatings or ink compositions, practically useful pigments exhibiting a clear color tone and high tinting strength are generally composed of fine particles. However, when the fine particles of pigments are dispersed in a non-aqueous vehicle such as an offset ink, a gravure ink, a resist ink for a color filter, an inkjet ink and a coating, it is difficult to obtain a stable dispersion. For this reason, a variety of problems are caused which exerts significant influences on fabricating jobs and the value of a product obtained.
For example, dispersions containing pigments formed of fine particles often show high viscosity and it is difficult to take out a product from a dispersing machine and to transfer it from the dispersing machine to a tank or the like. In worse cases, the product undergoes gelation during storage and becomes difficult to use. Further, when such a dispersion is mixed with a different kind of pigment, a coated matter sometimes suffers uneven coloring or a remarkable decrease in tinting strength because of a phenomenon such as segregation or settling due to the agglomeration of the pigments. In addition, with regard to the surface of coating film of the coated matter, a poor state such as a decrease in gloss or poor leveling occurs in some cases. Furthermore, while not directly related to the dispersing of pigment, some organic pigments suffer a phenomenon which accompanies a change of the crystal condition of the pigments. That is, with regard to an offset ink, a gravure ink, an ink for a color filter, a coating or the like, pigment crystal particles which are energetically unstable in a vehicle change in their size or shape and move to a stable state, so that the commercial value of a coated matter thereof is impaired by a remarkable change in hue, a decrease in tinting strength, the occurrence of coarse particles, etc., in some cases.
Pigments used for color filters or inkjet printings are required to have high heat resistance and weather resistance. The pigments to be used are phthalocyanine pigments, diketopyrrolopyrrole pigments, quinacridone pigments, isoindolin pigments, isoindolinone pigments, benzimidazolone pigments, quinophthalone pigments, azo metal complex pigments, etc. Of these, an azobarbituric acid nickel complex, i.e., C.I. Pigment Yellow 150 is a pigment having high tinting strength and it is widely used at the present time. However, it is difficult to control particles of the above pigment when it does not have a compound to be included. In this case, strong agglomeration occurs or its particles become uneven so that it is a difficult-to-disperse pigment. Further, it is known that the above pigment has crystal water and it is thermally unstable.
JP-A-58-52361 and JP-A-2000-119544 propose to improve dispersing by adjusting a pH by allowing an azobarbituric acid metal complex to include a compound, particularly preferably melamine. This method improves weather resistance and heat resistance. However, a pigment including melamine alone is still a difficult-to-disperse pigment and, in addition, large variation in quality is found in a batch production. Therefore, for dispersing the above pigment in a vehicle, it is required to disperse the above pigment mechanically by applying considerable energy or to concurrently use a so-called pigment derivative having a basic or acidic group introduced in a pigment structure. In each case, the cost required increases. In particular, the latter case has an adverse effect on resistance properties in some cases.
Further, it is known that dispersing is improved by adding a metal carboxylate or a surfactant when a stable pigment dispersion is desired. However, a considerable amount of the carboxylate or surfactant is required for obtaining a necessary effect. In some cases, the addition thereof causes a decrease in heat resistance or weather resistance, an increase in migration, etc., so that some properties, which are required when used to prepare a printing ink, a coating, an inkjet ink or a resist ink for a color filter, are lost. It is urgently desired to obtain a pigment dispersion excellent in dispersing without losing various resistance properties and it is accordingly required to improve the dispersibility of pigment.